In a W-CDMA (Wide band-Code Division Multiple Access) system, which is a typical wireless communication system, HSDPA (High Speed Downlink Packet Access), which is a high-speed packet transmission system for a downlink, and EUDCH (Enhanced Uplink Dedicated Channel), which is a high-speed packet transmission system for a uplink, have been standardized. According to these packet transmission systems, a base station carries out scheduling, and only when data transmission/reception is performed, allocates wireless resources to a plurality of mobile stations according to time multiplexing or code multiplexing, resulting in the enhanced efficiency of use of the wireless resources. This W-CDMA system will be briefly explained by employing the accompanied drawings.
FIG. 1 is a schematic network configuration view illustrating one example of the W-CDMA system. Herein, a plurality of base stations 11 and 12 are connected to a base station controller 10, and the base station controller 10 can be further connected to an external network. Each of a plurality of the base stations can accommodate a plurality of mobile stations. Herein, it is assumed that mobile stations 21 and 22 are connecting to the base station 11, an mobile station 24 is connecting to the base station 12, and a mobile station 23 is in a soft handover (referred to as SHO where deemed appropriate) and therefore is connecting to both these base stations 11 and 12.
Each of the mobile stations 21 to 24 uses an uplink/downlink dedicated channel (DPCCH: Dedicated Physical Control Channel) for transmission/reception of a control signal to/from its corresponding base station. In addition hereto, the mobile stations 22 and 24 are performing data reception using the HSDPA, and the mobile stations 23 and 24 are performing data transmission using the EUDCH. The mobile stations 22 and 24, which are performing data reception by using the HSDPA, uses HS-SCCH (High Speed-Shared Control Channel) and HS-PDSCH (High Speed-Physical Downlink Shared Channel) for reception, and HS-DPCCH (High Speed-Dedicated Physical Control Channel) for transmission. Further, the mobile stations 23 and 24, which are performing data transmission by using the EUDCH, uses E-HICH (Enhanced-Hybrid ARQ Indicator Channel), E-AGCH (Enhanced-Absolute Grant Channel), and E-RGCH (Enhanced-Relative Grant Channel) for reception, and E-DPCCH (Enhanced-Dedicated Control Channel) and E-EDPCH (Enhanced-Dedicated Data Channel) for transmission. In other words, a mobile station as well, which is executing the HSDPA or the EUDCH, uses a channel called DPCCH for transmission and reception, apart from channels for data transmission and reception. The DPCCH is used to transmit a pilot signal, which is used for channel estimation in order for mobile station and base station to secure synchronization and to carry out demodulation, and a TPC (Transmission Power Control) signal, which is a control signal for a closed-loop power control.
A closed-loop power control is performed for the transmission power of the DPCCH so that the quality of the DPCCH becomes closer to a target quality (herein, a target SIR (Signal to Interference Ratio)). For example, in the transmission power control of the DPCCH for a uplink, the base station compares an actually received SIR of the DPCCH with the target SIR being decided by the base station controller 10, and when the reception SIR actually received is smaller than the target SIR, transmits a TPC signal instructing that the transmission power be increased through the DPCCH for a downlink. Otherwise, the base station transmits a TPC signal instructing that the transmission power be reduced. The mobile station increases or reduces the power for the DPCCH in accordance with the instruction of the TCP signal received through the DPCCH (Non-patent document 1).
Herein, in a case where the mobile station uses the DPCCH for transmission/reception to/from a plurality of the base stations, that is, where the mobile station is in a soft handover (SHO) state like the mobile station 23, the mobile station 23 receives a plurality of the TCP signals, and among the received TPC signals, if the mobile station receives at least one TCP signal giving an instruction to reduce the power, the mobile station is controlled to reduce its power. This is because during the SHO, communication can be carried out so long as any one of a plurality of the base stations meets a desired quality, and because increasing the transmission power to make a plurality of the base stations receive a sufficient quality leads to increased interference with another user, which is not favorable.
In this method, however, the base station having a minimum transmission loss occurring in a route to the mobile station can almost accurately receive a control signal being transmitted from the mobile station, whereas the base station having a large transmission loss occurring in a route to the mobile station more frequently fails in receiving a control signal being transmitted from the mobile station because the reception power of the control signal is small. Thus, an error in the transmission power control augments in the base station of which the transmission loss is large, which make it impossible to keep the downstream transmission powers of respective base stations equal to each other.
Thereupon, there has been proposed the transmission power control method called power balancing for enabling each base station to perform transmission with an almost identical power during an execution of the soft handover (Non-patent document 2).
Next, one example of the transmission power control for the downlink will be explained by making a reference to FIG. 2. FIG. 2 is a flowchart in which the base station decides the transmission power of the DPCCH for the downlink upon receipt of the TPC signal from the mobile station during an execution of the soft handover. Additionally, at the moment that a base station starts to perform the soft handover with a mobile station, a transmission power P is defined as a value of the last transmission power for its mobile station, namely remains unchanged if its base station is a serving base station that has performed transmission to its mobile station so far, and a transmission power P is defined as an initial value P0 if its base station is a Non-serving base station that has newly started transmission to its mobile station. Further, it is assumed that the controller notifies the number of the frame for starting the soft handover to the Serving base station and the Non-serving base station.
Further, in the following explanation, P(k) is a transmission power that is controlled in a slot k within an adjustment period, P_bal is an adjustment amount of a power balance per one slot within the adjustment period, P_bal(k) is an adjustment amount of the power balance in a k slot within the adjustment period, I is the number of frames, k is a number of the slot, and the frame is comprised of the predetermined number of the slots, and the following explanation is made on the assumption that the number of the slots within one frame is L. Additionally, 3GPP stipulates that the number of the slots within one frame is 15. In addition hereto, P_TPC(k) is an adjustment amount at the time of a k slot based upon the closed-loop power control, Tinit is an adjustment period, P_ref is a reference power offset value being decided by the base station controller, P_CPICH is a power value of a common pilot signal being transmitted by the base station, and r is an adjustment ratio of the power balancing.
At first, when a transmission power balance control message among a plurality of the base stations arrives from the control station, P_bal=0, I=0, and K=0 are reset as an initial value, respectively (step 101, step 102, and step 103). Herein, the mobile station notifies a TPC signal at a constant time period, and when this newly notified downstream control command has been received (step 104) and its TPC signal is a signal for giving an instruction for increasing the power (step 105), the transmission power increase/decrease amount P_TPC(k) in a k slot by the TPC signal is increased by the predetermined value ΔTPC (step 106), and when its TPC signal is a signal for giving an instruction for decreasing the power (step 105), P_TPC(k) is decreased by a predetermined value ΔTPC (step 107). On the other hand, when the TPC signal is not received, P_TPC(k) is defined as P_TPC(k)=0 (step 108), the operation proceeds to a step 109.
And, P_bal(k) is defined as P_bal(k)=P_bal (step 109), and the transmission power P(k) in a k slot is controlled so that an equation 1 holds (step 110).P(k)=P(k−1)+P_TPC(k)+P_bal(k)  Equation 1
Additionally, when k=0, P(k−1) assumes the value of the final slot of the previous frame.
Continuously, k is incremented by 1 (step 111), and it is determined whether K has become L slot (step 112). If K is not L slot, the operation returns to the step 104, and the foregoing process is continued. On the other hand, when K becomes L slot, I is incremented by 1 (step 113). And, it is determined whether I=Tinit, that is, the adjustment period has expired (step 114).
When I=Tinit does not hold, the operation returns to the step 103, and the foregoing process is continued. On the other hand, when I=Tinit holds, a power P_init transmitted in the final slot of the adjustment period is assumed to be P(k−1) (step 115). And, so as to compute P_bal that is employed in the next adjustment period, a total amount of P_bal within the adjustment period is obtained from an equation 2 (step 116).Sum P_bal=(1−r)*(P_ref+P_CPICH−P_init)  Equation 2
Continuously, the value obtained by multiplying Tinit by Lslot, that is, the value obtained by dividing SumP_bal by a total slot number in the adjustment period is assumed to be P_bal of the next adjustment period (step 117), the operation returns to the step 102, and the foregoing process is continued.
Such a power balancing is stipulated by the equation 1 and the equation 2, and is stipulated by 3GPP (3rd Generation Partnership Project) as well that stipulates a W-CDMA Standard (Non-patent document 1 and Non-patent document 2).P(k)=P(k−1)+P_TPC(k)+P_bal(k)  Equation 1Sum P_bal=(1−r)*(P_ref+P_CPICH−P_init)  Equation 2
However, 3GPP does not specifically stipulate the adjustment method within the adjustment period. That is, as described above, Sum Pbal may be equally divided for adjustment so that the transmission power is adjusted only by a constant value in each slot within the adjustment period in some cases, and may be divided in a lump or several times for adjustment only in a predetermined slot within the adjustment period in some cases.
By the way, in the foregoing 3GPP, there has been proposed the technology (which is called DPCCH Gating) of transmitting the control signal only in limited slots within the predetermined time period without performing continuous transmission of the control signal such as the TPC signal and the pilot signal in the DPCCH for the uplink in a case where the data transmission to the mobile station in question or the data transmission from the mobile station in question has not been performed in the HS-PDSCH and the E-DPDCH (Non-patent document 3). Herein, the so-called “no data transmission/reception to/from the mobile station” signifies that no transmission/reception of a signal has not bee performed between the mobile station in question and the base station over a predetermined time in at least one channel of the HS-PDSCH and the E-DPDCH. Such a situation occurs, for example, during the time (reading time) that a user of the mobile station, who is performing a Web perusal by using the HSDPA, is reading a downloaded Web page.
Upon explaining this technology by employing FIG. 3, the control signals such as the TPC signal and the pilot signal are continuously transmitted in a slot within the frame of the DPCCH when the data transmission is being performed. Such a situation is called a normal mode. On the other hand, when the data transmission is not performed, the control signals such as the TPC signal and the pilot signal are transmitted intermittently only in x slots (x<N) out of N slots of the DPCCH, or transmitted off and on, and are not transmitted in the slots other than these slots. Hereinafter, such a situation is called an intermittent transmission mode. Additionally, in FIG. 3, x slots in which the control signal is transmitted are listed on the assumption that they are continuous slots; however it is not always necessary that they are continuous.    Non-patent document 1: 3GPP TS25.214 v6.7.1 (2005-12) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD)(release 6)    Non-patent document 2: 3GPP TS25.433 v6.8.0 (2005-12) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; UTRAN Iub interface Node B Application Part (NBAP) signaling (release 6)    Non-patent document 3: 3GPP TR25.903 v0.3.0 (2006-02) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Continuous Connectivity for Packet Data Users; (release 7)